The prior art will be described in terms of resin coated sand used in the Shell Process employed by the metal casting and foundry industry. The shell process was developed in Germany during the Second World War, and the process was used to produce molds for mortars, artillery shells and other projectiles. The Germans attempted to keep the process secret after the war; however, the process was discovered by allied investigators who placed the process in the public domain as war booty which then provided the foundry industry with a revolutionary process.
The Shell Process (also known as the Croning or C Process) is used to produce hollow light weight molds and cores for pipe hubs, cores, crank shafts, intake manifolds for engines, etc. In fact, more foundries utilize the shell process, to produce resin sand cores and molds, than any other process. The process is extensively applied worldwide.
The original process blended raw sand with powdered phenolic resin and powdered hexamethylenetetramine (a curing agent) which was gravity fed into a preheated pattern. The heat melted the resin and hardener to fuse the sand. After a suitable thickness of sand was obtained, the unactivated sand was dumped from the pattern, leaving the hollow core sand mold. As time went by, the process was improved by pre-coating the sand with the required ingredients (resin-hardener-wax-fillers-etc.) at a sand facility. The “foundry sand” is then sold as a free-flowing product to foundries (or foundries produce their free-flowing product).
The current state of the art uses batch mixers to coat substrates (minerals, ceramics, etc.) with a resin(s) and other ingredients. That is, sand is preweighed, heated to the desired temperature and transferred into a batch mixer. Resin(s) and additives are then added sequentially and held in the mixer until the material has reached the required cure stage or begins to break down into smaller agglomerated clumps of sand and resin. The mixture is then dumped and the cycle is repeated.
U.S. Pat. No. 4,439,489 to Johnson et al. discusses several processes for coating proppants (a substrate used in the oil industry) all of which use a batch process. U.S. Pat. No. 4,090,995 to Smillie describes another batch process used to coat sand for use in shell molds. It is interesting to note that the resin acts in proppants and shell sands in a similar manner—that is the resin acts to hold the substrate in a fixed shape, or to strengthen the substrate. Thus, the techniques used to coat proppants or industrial sand is similar.
It is well known that batch processes are time consuming. For example, the assignee currently uses 1000-1200 pound [455-546 kg] pug mixer with included mixer paddles at its Bridgman, Mich. plant. (See FIG. 1) The typical batch mixer cycle is 3-5 minutes depending on product formulation. Approximately 15 seconds of non-productive time is experienced as each batch is discharged, and a new sand charge is added to the mixer. Thus, the mixer can only produce 1200 pounds [546 kg] times every 3-5 minutes or around 144,000 pounds [65,455 kg] per shift. It is believed that if the process is modified to be a continuous mixer process, the mixer would be capable of operating at significantly higher production rates.
There are other problems with the batch process explained above. Heated sand is dropped in the batch mixer, and the required resins and other ingredients are added to the mixer with the paddle stirring the mix. Minutes into the mixing process, hexamethylenetetramine (hexa—the curing agent or hardener) solution is added along with water and wax. At this stage the mix agglomerates (goes from a free mixing sand to a material that looks like stiff bread dough), thus reducing the probability of an even coat of the particles. The mixing process continues thereby breaking up most of the agglomerated mixture. The portion of the mixture that does not break up is sent to a roll crusher (or similar device) that breaks up chunks of particle mix eventually producing a free flowing product. It should be remembered that the foregoing example is for a 1200 pound [546 kg] batch type mixer. Various sizes of batch type mixers will yield different production rates.
There are several manipulative stages involved following the batch mix process used to create a free flowing product, and these stages require considerable transfer of the material through various material handling systems and various physical levels within the process plant thereby expending considerable energy. The process described above is for a single coating and often (particularly for “frac-sand”) the substrate is coated a second and even a third time.
The concept behind a resin coated substrate is to obtain a uniform coating on each particle. In order to coat a particle, the substrate is heated to obtain a surface temperature hot enough to melt the resin while not heat soaking the particle. The curing agent is then brought into contact with the heated particle. If the particle remains hot, then the resin will cure (referred to as the “C-stage”); however, if the particle is quenched, then the resin will only partially cure (referred to as the “B-stage”). Thus, in the coating process the use of solution curing agents (hexa in solution) is paramount. The water in the solution removes the heat from the particle. Some resins require a higher melting temperature, thus, more heat must be removed and additional water is added after adding the curing agent. In some cases additional cooling is required and air or a cooling water jacket is be used.
Thus, there remains a need to convert batch resin coating processes to continuous processes thereby increasing productivity and reducing energy use while maintaining or even improving product quality.